This invention is particularly useful in, but not restricted to, a thermal printer such as described in U.S. patent application Ser. No. 457,593, filed Dec. 27, 1990 in the names of S. Sarraf, et al., and assigned to the same assignee. This printer thermally prints an image on a receiver element (e.g., a slide transparency) by scanning a laser beam across a dye donor element sandwiched against the receiver element. The laser beam is modulated by input data corresponding to the image to be printed, and the beam is swept across the receiver element a line at a time by an electro-mechanical galvanometer which rotates a mirror to deflect the laser beam linearly. To obtain sufficiently high resolution, such as is needed in slide transparencies to give a desired degree of sharpness in the projected image, the laser beam is focused through a f-.nu. lens to a spot size of only 7 microns (.mu.m) onto the element being printed. The spots are written at a pitch of 6 microns to obtain a print resolution of about 4000 dots per inch. To produce an optical image on the slide transparency as nearly perfect as possible, the laser "spots" must be positioned with extreme accuracy (within a micron). This accuracy must be absolutely maintained and repeated during millions of cycles of operation. Moreover, because the image on the slide is being produced sequentially by "writing" a line at a time (rather than all at once), it is necessary that the galvanometer mirror scanning motion be very linear and fast, that it be free of spurious vibrations, and that there be a minimum of idle time during re-positioning (i.e., re-setting) of the galvanometer for the next line scan. These requirements impose difficult-to-meet constraints on the galvanometer mechanism, on its mode of operation, and on its control and drive circuitry.
In the above-identified U.S. patent applications there is described a greatly improved galvanometer mechanism which is incorporated by reference into this patent application. This improved galvanometer has a moving magnet armature inside a stationary electric drive coil which is partially surrounded by an iron pole piece. A low inertia mirror having an optically flat reflecting surface is rigidly mounted on the armature and is adapted to be rotated angularly plus and minus a given amount (such as 7.8 degrees) on either side of a center or rest position. The mirror is placed at an angle of 45 degrees in the path of the laser beam of a thermal printer and when the mirror is rotated from one angular extreme to the other, the beam is scanned linearly across a receiver element being printed. The moving armature of the galvanometer and its mirror are precisely supported by a unique upper and lower spring pivot arrangement, which provides for ease of rotation around a defined axis, but which eliminates friction and strongly resists spurious movements and/or vibrations along other axes or in other directions. The pivot arrangement also provides very accurate and predetermined spring torque which, when the galvanometer and its mirror are at rest, holds them in a centered or zero position. The present invention provides an improved method of, and electrical circuit for, repeatedly scanning such a galvanometer and its mirrored beam along a line. The invention provides substantially greater accuracy and linearity in scanning and also provides for re-positioning the galvanometer and mirror for the next scan very quickly and precisely and with virtually perfect freedom from unwanted oscillations and overshoot than do previous drive arrangements. The present invention is not limited solely to the galvanometer mechanism disclosed in the above-mentioned U.S. patent applications.